2004 chevy 3500 dual tank gage problem

The fuel gage problem is either from a bad ground to the gage or the sending unit in the tank is bad. They will normally read a half tank or less when the car is full if the sending unit goes bad. I would put a scanner on the computer and check for any codes set.

Its very simple. If the gauge is showing half full then fill the tank and see how much fuel it takes. If the amount you add is approximately 50 percent of the fuel tank capacity then you know the fuel gauge is accurate . In that case the computer is giving an inaccurate reading and should be ignored. All you need is an accurate fuel gauge - it does not matter a row of beans what the computer says if the fuel gauge is reading correctly.

Most likely a defective fuel level sending unit, located inside the tank connected to fuel pump assembly. Theres a possibility that you have a ground issue. So check that first as its easier. If the grounds check out then you have a few options. 1. Pull the fuel pump assembly and clean off the fuel level sender(tons of tutorials online for this). This fixed my problem for 10 months until my pump went out. 2. Replace just the fuel level sender. 3. Replace the entire fuel pump assembly (which should include sender and fuel pressure sensor). And a couple notes. If you have around 100K or more miles then DEFINITELY change the entire assembly. It beats having to do all the work again in the near future. No matter what route you go be sure to replace the fuel pump tank gasket.

It sounds like you need to replace the sending unit which is located in the fuel tank and is part of the pump unit. Some you can buy just the sending unit and others it comes as a module which means the pump and sending unit come together. Not sure what yours has. When changing I find it easier to remove the bed that to drop the fuel tank.

If this was not a problem before the pump replacement then the sender in the tank may of faied or is contaminated with sulfer which is corrosive to the silver electrical contacts.

Do a search on sulfur corrosion and fuel sender units. Gm has big problem with this. Its possable the stress from removing the old and installing the new pump created an issue with contacts. If this is the case you will now need a sender unit.

tank switch on dash could be bad, also selector valve could be bad. are the fuel gauges accurate front or rear. like you say the rear reads full, is it?? I would try the switch on the dash first.

Normally it's just a matter of removing filler tubes from side, disconnect fuel lines & electrical connections at fuel pump/sender, undo the 2 straps, & lower it down.

take the wire off that goes to the tank and ground it. if the Gage moves then it is the float if the Gage stays there then it is the Gage.

Is......the problem--"the gas gage reads empty when there is fuel in the tank. OR, The gage moves when the key to turned off ?

If it's the first,
You will need to test the gage and through process of elimination, determin if the Sending unit inside the fuel tank has gone Bad.

If its the second ? - Electrical gauges will move when powered down. Normal

Testing the gauge may not be easy - just depends on your ability.
It is a simple 2-wire gauge circuit, when disconnect (usually need the fuel tank) the gage reads in one direction (either full or empty)
then when shorted-the gage reads in oppisite direction.

Of course you must select the correct wires or you may short out the wrong circuits!! Most GM vehicles use a purple wire & black wire.

A quick method I have used to check the in-take part of the gauge is this.
1st. know there is gas - 1/2 tank works best.
2nd driver's door open, key in run position, engine not running.
3rd gently lift up on the door then let if come back down, gently.
just enough to cause the gas to slosh around.
4th - keep an eye on the gas gage, a bad sending unit in the take will show up as a "twitchy" needle.

Of course, if the gage never leaves empty - sloshing won't work.
I have a known good sending unit I can plug into the vehicle and simulate different levels of fuel - if the gas gage works, I know the problem is in the tank.

Let us know.

Dave

Yes it is all one unit, and yes that is what the problem is. don't use aftermarket parts, use OEM, (Delco Remy) they are much better quality.